Tessa Halterman was aboard Sound Transit bus No. 510 on her way to work in downtown Seattle when it came to a sudden stop Friday morning in the middle of an Interstate 5 express lane just south of Northgate.

After the driver stepped out to check the back engine, Halterman and the woman next to her noticed an intense burning smell, followed by two loud pops.

Then the driver ran back to the crowded bus and yelled at passengers, some standing in the aisles, one in a wheelchair, to evacuate immediately.

About 30 seconds later, the back of the bus exploded in flames, Halterman, said.

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“I feel crazy saying this, but there were flames literally 30 feet in the air, and the entire back part of the bus is just totally gone,” said Halterman, a lawyer at Robinson Tate law firm downtown.

No one was injured in the explosion, which happened at about 6:45 a.m. and closed southbound I-5 for almost an hour. Authorities are investigating the cause of the fire.

After the explosion, passengers began running down the express lane to get as far from the bus as possible.

A few stayed to help carry a woman in a wheelchair off the bus.

“That bus driver deserves some sort of recognition, because if he hadn’t handled the situation the way he had, it would have been a lot worse,” Halterman said.

Other drivers who had helped flag down the bus so it would stop also helped save passengers from injury or worse, said Sgt. Dan Atchison of State Patrol.

Seattle firefighters had the fire out by 7:38 a.m., said Fire Department spokesman Kyle Moore. Passengers were picked up by another bus, according to Sound Transit spokeswoman Kimberly Reason.

The southbound mainline of I-5 reopened at 7:45 a.m., State Patrol Trooper Julie Judson said. Two of the express lanes reopened just after 9:30 a.m., but traffic delays remained throughout the morning. The freeway wasn’t completely cleared until about 11 a.m., Atchison said.

State Patrol detectives found the fire was mechanical. Atchison said friction from a locked brake likely sparked the fire, but Sound Transit will officially determine the nature of the mechanical failure.

“It could be any number of things — for example, it’s possible moisture in the brake lines could have caused the brakes to freeze, which would have caused enough friction between the brake pad and brake drum to create the conditions that led to the fire,” Reason wrote in an email.

Cathy van Winkle, a teacher at Wing Luke Elementary on Seattle’s Beacon Hill, said it took her an hour and 45 minutes to drive to work from her home in Shoreline, using Greenwood Avenue North to go around the blockage. It normally takes her about 40 minutes.